From the Economist of 12/3/11, p. 43, in “Marijuana in California and Colorado: Highs and laws” [the magazine is fond of jokey titles], after a long first paragraph about medical marijuana boom in Colorado:
While it is allowed in some form in 16 states and Washington, DC, Colorado is the leader in trying to make medicinal pot a legitimate business.
Now, (medical) marijuana is highly topical when this sentence comes along in the discourse. so that’s almost surely the referent of the subject pronoun it in the initial subordinate clause. Nevertheless, I expected this pronoun to be cataphoric, preferably with its referent picked up by the subject of the main clause — but that’s Colorado (where Colorado is paired with 16 states and the District of Columbia), and not a NP referring to marijuana. So I had a brief moment of unfulfilled expectation that wasn’t ironed out until medicinal pot came along, embedded within the main clause.
My reaction to this explicit pronoun subject it is much like many people’s reaction to zero subjects in initial sentence adverbials, in initial SPARs (subjectless predcative adjuncts requiring a referent for the missing subject). Sometimes the referent is given right there in the preceding context, but still we expect the zero to be cataphoric, preferably to the subject of the main clause.